Associated with recent small-sized high-performance mobile phones and electronic devices, nonaqueous electrolyte batteries such as lithium secondary batteries exhibiting high energy density with high voltage draw attention and are widely used as the power source thereof.
Conventionally, it is known to use lithium titanate as an active material for a negative electrode in such lithium secondary batteries and lithium ion batteries (see, Patent Document 1). Further, it is known to substitute a part of the element of the lithium titanate with Fe (see, Patent Document 2), with Cu (see, Patent Document 3) and with other transition metals and the like (see, Patent Documents 4 to 6).
Patent Document 1: JP 06-275263-A
Patent Document 2: JP 2001-185141-A
Patent Document 3: JP 2001-250554-A
Patent Document 4: JP 2004-235144-A
Patent Document 5: JP 10-251020-A
Patent Document 6: JP 2000-156229-A
Particularly, Patent Document 4 describes an invention of “a negative electrode active material for nonaqueous secondary battery having a lithium transition metal composite oxide having a spinel structure containing an alkali metal and/or alkali earth metal.” (claim 1). However, “a contained alkali metal and/or alkali earth metal stabilizes a crystalline structure of the lithium transition metal composite oxide having a spinel structure, thereby possibly improving cycle characteristics.” (paragraph [0022]). As there is described that “lithium titanate represented by a general formula LiaTibO4+c (where M denotes at least one element selected from the group comprising one or more transition metals except titanium, 2, 13 and 14 group elements of the periodic table, halogen elements and sulfur, a+b denotes a number satisfying 0.8≦a+d≦1.5, b denotes a number satisfying 1.5≦b≦2.2, and c denotes a number satisfying −0.5≦c.≦0.5) improves cycle characteristics, stability and reliability.” (paragraph [0029]), the invention described in the Patent Document 4 has absolutely an object of improving cycle characteristics and the like by adding less than or equal to 0.1 mol of an alkali earth metal such as Mg or the like to lithium titanate and has no object of reducing voltage variations during discharge (of improving voltage flatness). Moreover, as lithium titanate satisfying the above-described general formula, there is described a negative electrode active material of Li1.26Ti1.72Mg0.02 in which Ti-content is high and Mg-content is low (when Li and Mg contain the above-described amounts, Ti becomes tetravalent (Ti4+) in an amount of 1.675 mols, and thus it becomes partially trivalent (Ti3+) in an amount of 1.72 mols) (see example 1). However, as described below, this Mg-containing lithium titanate is inferior in electrochemical capacity.
Patent Document 5 describes an invention of “metal-substituted lithium titanate represented by a general formula LixMyTizO4 (where M is a metal having a valence of two or more, and 0.5≦(X+Y)/Z≦2), in which a part of a lithium component of lithium titanate is substituted by a metal having a valence of two or more” (claim 1), and magnesium is described as a metal having a valence of two or more (claim 2). However, as it is described that “doping and dedoping of lithium ions are facilitated by substituting a part of a lithium component with a metal having a valence of two or more, thereby enabling characteristics such as battery capacity and the like to be improved when used as an electrode for a lithium battery” only a part of the lithium component is substituted with magnesium, and no titanium component is substituted therewith. Further, this invention has no object of reducing voltage variations during discharge (of improving voltage flatness).
Moreover, it is further known to use a spinel compound represented by a composition formula: Li4-xMgxTi5O12 (0≦x≦1) as a negative electrode of a lithium battery (see Non-Patent Document 1).
Non-Patent Document 1: Journal of The Electrochemical Society, 148(1) A102-A104 (2001)
The spinel compound in the Non-Patent Document 1 is one in which only a part of Ti of Li4Ti5O12 (lithium titanate) is substituted with Mg. Although it is described that conductivity and high rate discharge characteristic can be improved by using this Mg-containing lithium titanate, it is not described that voltage variations during discharge can be reduced (voltage flatness can be improved). Moreover, in the above-described composition formula, Mg is a divalent representative element (Mg2+), and since only a part of monovalent Li component (Li1+) is substituted with Mg having the same number of moles and no tetravalent Ti component (Ti4+) is substituted with Mg, a part of Ti becomes partially trivalent (Ti3+)
On the other hand, a spinel compound represented by a composition formula: Li[Li(1-x)/3CrxTi(5-2x)/3] is further known as a negative electrode active material of a lithium ion battery (see Non-Patent Document 2).
Non-Patent Document 2: Journal of Power Sources, 125 (2004) 242-245
The spinel compound in the Non-Patent Document 2 is one in which a part of Li— and Ti components of Li4/3Ti5/3O4 (lithium titanate) is substituted with Cr. Although it is described that diffusion coefficient and high rate discharge performance are improved by using this Cr-containing lithium titanate, it is not described that voltage variations during discharge can be reduced (voltage flatness can be improved). Moreover, in the above-described composition formula, Cr is trivalent (Cr3+) and Ti is tetravalent (Ti4+). However, since Cr is a transition metal, the valence number of Cr will vary due to electrochemical reduction of lithium titanate when such lithium titanate is used as a negative electrode active material of a battery. In order to provide a negative electrode containing lithium titanate with an excellent discharge capacity in the potential flat part, it is important that tetravalent Ti (Ti4+) and trivalent Ti (Ti3+) coexist in lithium titanate contained in the negative electrode which has been charged for the first time after the composition of the battery and it is further desired that trivalent Ti (Ti3+) exists more than tetravalent Ti (Ti4+). However, so the valence number of Cr varies due to electrochemical reduction of lithium titanate, trivalent Ti (Ti3+) is produced insufficiently, thereby making it difficult to make an above-described effect.